1. Field of the Invention
The present invention relates to a surface acoustic wave device for use as a band-pass filter, and a resonator, for example, and a method of producing the same, and more particularly, to a surface acoustic wave device which includes an interdigital electrode made of tantalum, and a method of producing the same.
2. Description of the Related Art
Conventionally, surface acoustic wave devices have been widely used as band-pass filters and resonators. For surface acoustic wave devices for such uses, very good high frequency characteristics are critical for desired operation.
Further, cost-reduction is urgently required for the surface acoustic wave devices, as well as other electronic components.
In the surface acoustic wave device disclosed in Japanese Examined Patent Publication No. 61-45892, an interdigital electrode made of gold is disposed on a X-rotated, Y-cut quartz substrate in such a manner that the X axis and the surface acoustic wave propagation direction define an angle of about 90°.
Further, Japanese Unexamined Patent Application Publication No. 10-247835(Japanese Patent Application No. 9-61731) discloses a surface acoustic wave device provided with an interdigital electrode with a two-layer structure which contains an aluminum layer as an underlying layer and a tantalum layer laminated to the aluminum layer.
In the case where a narrow-band-pass filter includes a surface acoustic wave device, it is desired that uneveness in the center frequency of a surface acoustic wave device is minimized. Accordingly, for production of a surface acoustic wave device for use as a narrow-band-pass filter, it has been conventionally required to finely adjust the frequency of the surface acoustic wave device after the production process is completed.
A typical method of finely adjusting the frequency as described above is such that the fine adjustment of frequency is carried out by dry-etching an interdigital electrode with plasma using CF2+O2 (Appl. Phys. Lett, 39(1), p.40 (July, 1981)).
For example, in production of a surface acoustic wave device including a quartz substrate, ordinarily, the shift of the center frequency is about 300 ppm. According to the above-described fine adjustment method, the frequency can be changed by a maximum of about 500 ppm. It is estimated that finally, the shift of the center frequency can be suppressed to be within 50 ppm.
Generally, the operation frequency of a surface acoustic wave device is determined by f=v/λ (v=propagation velocity of surface acoustic wave, λ=wavelength of surface acoustic wave). The wavelength λ depends on the configuration of an interdigital electrode. Ordinarily, the interdigital electrode is formed by a fine processing technique using photolithography. Generally, the above-mentioned fine-processing is performed by a combination of a thin-film forming technique such as a chemical deposition method, e.g., vapor deposition, CVD, or other suitable method, and a physical deposition method, e.g., sputtering, with an etching process or other suitable method, and further by repeating these techniques, if necessary.
When gold is used as a material for forming an interdigital electrode as in the surface acoustic wave device described in Japanese Examined Patent Publication 61-45892, the gold is formed into a film by a chemical deposition method in general. However, when an electrode made of gold is formed on a piezoelectric substrate by chemical deposition, there arises the problem that the adhesion of the electrode made of gold to the piezoelectric substrate is insufficient. Accordingly, for improving the strength of adhesion between the interdigital electrode and the piezoelectric substrate, it is necessary to form an electrode layer of chromium or other suitable material as the underlying layer for the electrode of gold. Thus, this greatly complicates the production process. Moreover, gold is expensive, which increases the material cost, and also, the cost of the surface acoustic wave device.
On the other hand, as disclosed in Japanese Unexamined Patent Application Publication No. 8-125485, it has been proposed that tantalum is used as an interdigital electrode material. Ordinarily, tantalum is film-formed by a physical deposition method such as sputtering, since tantalum is a metal having a high melting point. However, the crystal structure of a tantalum film formed by such a method is that of β-tantalum, and the resistivity is relatively high, that is, 180 μΩ·cm. Accordingly, the characteristics of the surface acoustic wave device, especially the insertion loss, tend to be deteriorated, since the electric resistance of the electrode is increased.
Accordingly, as disclosed in Japanese Unexamined Patent Application Publication No. 10-247835, a method of forming an aluminum layer with high conductive properties as an underlying layer for the electrode layer made of tantalum has been proposed. However, for formation of the interdigital electrode, it is necessary that plural metal layers are laminated. This causes the production process to be complicated and expensive.
As described above, it is difficult to adjust the shift of a frequency by at least 500 ppm in the method of finely adjusting the frequency of a surface acoustic wave device by dry-etching utilizing plasma. This is because the plasma damages a piezoelectric body, resulting in deterioration of the insertion loss of the surface acoustic wave device.
Moreover, in order to perform the above-described fine adjustment of frequency, it is necessary that the fine adjustment of frequency is conducted for every substrate. Accordingly, there is the problem that the yield ratio is reduced when the film-thickness distribution of the interdigital electrode is uneven.